Microstructural and mechanical properties of poly(sialate-siloxo) networks obtained using metakaolins from kaolin and halloysite as aluminosilicate sources: A comparative study

Highligths

• Metakaolins from kaolin and halloysite clays were prepared for producing geopolymer cements.

• The metakaolins from kaolin have a plate shape with coarse particles.

• Whereas metakaolin from halloysite have spherical morphologies with the fine particle sizes.

• The spherical shape and smaller particle sizes of metakaolin from halloysite creates the nucleation sites.

• Compressive strengths of geopolymer using metakaolin from halloysite are higher compared to those from kaolin.

Abstract

This work focuses on the comparison between the mechanical and microstructural properties of poly(sialate-siloxo) networks based on metakaolins from halloysite and kaolin. Poly(sialate-siloxo) networks were prepared using three metakaolins as aluminosilicate sources. Sodium waterglass from rice husk ash and commercial sodium waterglass were used as chemical reagents. The obtained results showed that metakaolins from kaolins have plate shapes with coarse particle sizes whereas the one from halloysite has a spherical morphology and smaller particle sizes. The IR spectra of poly(sialate-siloxo) networks from calcined halloysite indicate the higher value of the wavenumber of the main band. The XRD patterns of all poly(sialate-siloxo) networks show the broad hump structure with higher intensity between 18 and 40°(2θ). The XRD patterns of poly(sialate-siloxo) networks show the band of the unreacted metakaolin at about 20.45°(2θ). This band is more pronounced on the XRD patterns of geopolymer cements from calcined halloysite. The obtained poly(sialate-siloxo) networks based on metakaolins from halloysite and kaolin have a compact, homogenous and denser microstructures. The compressive strength values of the poly(sialate-siloxo) networks using calcined kaolin are ranging from 58.43 to 66.52 MPa whereas those using calcined halloysite are between 72.29 and 88.50 MPa. The compressive strength values of poly(sialate-siloxo) networks using calcined halloysite are higher compared to those from calcined kaolin. The higher compressive strength values of the geopolymer cements from calcined halloysite could be attributed to the fine and spherical particle sizes of calcined halloysite. This implies that the shape and the fine particle sizes of the raw materials influence the properties of the poly(sialate-siloxo) networks. Metakaolin from halloysite can be used as an aluminosilicate source for producing poly(sialate-siloxo) network with higher mechanical properties.

Introduction

The most important minerals contained in the kaolin group are kaolinite and halloysite. Both minerals are dioctahedral 1:1 layer silicates and these minerals are denoted hydrated aluminosilicate materials. Halloysite is a natural nanoscroll material which exists in two forms namely the hydrated and dehydrated forms with the chemical formulas Al₂Si₂O₅(OH)₄.2H₂O and Al₂Si₂O₅(OH)₄, respectively (Nicolini et al., 2009). This implies that halloysite has a similar composition like kaolinite except that it contains additional water molecules between their layers. This justifies the fact that the basal distance of the hydrated form is around 10 Å whereas the one of dehydrated form is around 7 Å. The dehydrated form of halloysite also called metahalloysite is identical to the one of kaolinite. This means that the identification of halloysite 7 Å is difficult (Brindley, 1980). The shape of the kaolinite particles is hexagonal plate whereas halloysite displays tubular, spherical and cone-shaped morphologies (Kitagawa, 1976). According to Senoussi et al. (2016), the tubular form is predominant in the structure of halloysite motivating to use it as an absorbent. According to Sudo (1953), halloysite possesses fine particles size (between 0.05 and 0.50 μm) and Hong et al. (2009) reported that kaolinite has a coarse-grained ranging from 1 to 2 μm in its structure. Nelson and Hendricks (1943) and Dyal and Hendricks (1950) reported that the value of the specific surface area measured by nitrogen and ethane adsorption is in the range 7–30 m²/g for kaolinite and about 45 m²/g for halloysite indicating that halloysite has a higher specific surface area compared to the one of kaolinite. Halloysite has been already used by several authors either as an adjuvant and an additive to alter the properties of Portland cement pastes and concretes. For example, Farzadnia et al. (2013) used it as an adjuvant to investigate the effect of halloysite on the properties of cement mortars. In this work, 1, 2 and 3% of halloysite was used to substitute the Portland cement. They reported that the compressive strength and gas permeability of the final product containing 2 and 3% of halloysite ameliorated the properties of mortar from Portland cement up to 56 and 24%, respectively. Whereas Zapała-Sławeta (2017) used metakaolin from halloysite as an additive, i.e., 0, 5, 10, 15 and 20% to substitute Portland cement CEM I 42.5R in order to study the effect of metakaolin from halloysite on the alkali-aggregate reaction in concrete. They demonstrated that it is possible to mitigate alkali-silica reaction and lower expansion of mortar bars with reactive aggregate to a safe level of not more than 0.1% at 14 days by adding 20% of metakaolin from halloysite to Portland cement. The literature indicates that geopolymer cements were mostly produced using fly ash, slag, metakaolin from kaolin, volcanic ash and so on. In our point of view, the aluminosilicate named halloysite could be used for producing geopolymer cements with higher mechanical properties. For example, Zhang et al. (2012) used this material as a secondary mineral to study its influence in kaolin on the properties of geopolymer cements. They concluded that halloysite present in kaolin improved the reactivity of the metakaolin and therefore have a positive effect on the properties of metakaolin-based geopolymer materials. Up to now, some researchers such as Zivica et al. (2014) used halloysite as a raw material for synthesizing geopolymer cements. In this work, the author's calcined halloysite at 650 °C for 4 h. They reported that the compressive strength of the hardening geopolymer cement paste cured at room temperature for 24 h and compacted in the fresh state with a uniaxial pressure of 300 MPa was 76.2 MPa whereas the one without compacted was 0.03 MPa. Kaze et al. (2018) also used metakaolin from halloysite for producing geopolymer cements. These authors used sodium waterglass containing sodium hydroxide with molar concentrations 8, 10, 12 and 14 M. The volume ratio sodium hydroxide/sodium silicate was kept constant at equal to 1/1. They reported that the maximum compressive strength is around 27.5 MPa. We assume that the properties of halloysite mentioned above worth particular attention especially for its use in the synthesis of geopolymer cements. Several researchers attempted to compare the properties of geopolymer cement using metakaolin and other aluminosilicate sources. For examples, Robayo-Salazar et al. (2016) describe the effect of metakaolin on the microstructure and compressive strength of geopolymeric systems based on natural volcanic pozzolans. Douiri et al. (2017) compared the structural and dielectric of geopolymers prepared with metakaolin and Tunisian natural clay. Sudagar et al. (2018) investigated the influence of cork waste residue on metakaolin-zeolite-based geopolymers. Belmokhtar et al. (2018) investigated the comparison between the structural and textural properties of geopolymer cement from industrial sludge and commercial kaolin. Bhardwaj and Kumar (2019) attempted to compare the properties of geopolymer and alkali-activated slag concrete comprising waste foundry sand. Marinkovi et al. (2017) determine the appropriate functional unit for concrete mixes with possible different performances and compare these impacts for different concretes. Despite these works, no research has been done on comparing the properties of geopolymer cement using metakaolins from kaolin and halloysite. Rice husk is one of the most widely available agricultural wastes in many rice-producing countries around the world. Burning of rice husk in ambient atmosphere leaves a residue, called rice husk ash (RHA). The ash content of around 85–97 wt% of amorphous silica (Rozainee et al., 2008). RHA is highly porous and lightweight, with a very high external surface area. The presence of a high amount of silica makes it a valuable material for use in industrial application. This low-value silica-rich waste was used by several researchers (Tchakouté et al., 2016a, Tchakouté et al., 2016b) for preparation of sodium waterglass.

The aim of this work is to compare the microstructural and mechanical properties of poly(sialate-siloxo) networks obtained using metakaolins from kaolin and halloysite as aluminosilicate sources. The chemical reagents used are a commercial sodium waterglass and a sodium waterglass from rice husk ash. The comparative study of poly(sialate-siloxo) networks using metakaolins from kaolinite and halloysite was done by the determination of their compressive strengths and apparent densities. The fragments of samples obtained after compressive strengths measurement were used for scanning electron microscopy (SEM) observations. The powders were used for identifying the crystalline phases and functional groups in the obtained geopolymer cements using XRD diffractometry and IR spectroscopy, respectively.